What Is a Volcano?

“Tanner’s Volcano”, a drawing by a child of about 10 years of age. (Oregon State University)

For most people, our first contact with volcanoes will probably have been through the medium of children’s stories; tales of fiery mountains that captured our childish imagination. Since our imagination as children tends to be small and friendly, volcanoes too become small and friendly, a nuisance at most but always exciting! Who has not made a picture in their mind of that mountain, burning away merrily like a firework for our amusement and delight?

A cut-away drawing such as this one will probably have been our first encounter with a more scientific or factual representation of what volcanoes are. (Kidzworld)

As we grow older and go to school, we are presented with a slightly more realistic image of what a volcano is. We learn that they are confined to certain, mostly far-away regions, that they possess a magma chamber and that they can be deadly. If we paid close attention, we may remember that there are two main types of volcano, the stratovolcano (from strata, layers) and shield volcano, and that there are two main types of eruptions; the explosive or “grey” and effusive or “red” respectively. We also learned that volcanoes can be active, dormant or extinct. But in our mind, we still retain that picture of the burning, fiery mountain and regard the it as being the volcano, one that possesses a magma chamber from where it fetches the fire.

Later in life, and usually because an eruption affects us directly or indirectly, volcanoes may capture our imagination anew and we want to learn more about them. Since we are adults and pc-literate, we search on the Internet and encounter a far different view on what volcanoes are which is reported to us bewildering array of unfamiliar terminology. One of the first ideas we have to shed is that the mountain is the volcano, it is only the visible, surface expression of one.

Three types of volcanoes from left to right (all images WikiMedia):– Sunset Crater, Arizona, a 340 m high monogenetic cinder cone– Mauna Kea, Hawaii, a 4,205 m high shield volcano– Koryaksky, Russia, a 3,456 m high stratovolcano overlooking the city of Petropavlovsk, Kamchatka

The second concept we have to grasp is geologic time and the sooner we do so, the easier will it become to grasp the concepts of Volcanology as the science of volcanoes is known. Where we humans concern ourselves with time and conceptualise it as expressed in hours, days, weeks, months and possibly years as decades and centuries are almost too long for us to grasp. Just like the bored child in the back seat, we have to stop asking “Are we there yet?” and learn patience. Geologic processes take time far beyond the scope of human affairs.

If we are to translate geologic time, our days and weeks are but geological seconds and minutes. When volcanologists warn that an eruption is “imminent”, it could happen the next day, days or even weeks and usually not in the next few minutes. When they say that a volcano is showing signs of unrest and is moving towards an eruption, that eruption may happen this year but is far more likely to occur one or more years, possibly even decades down the line if at all. Recently, a Colombian volcano, Volcan Chiles, moved from a state that must be considered at the very least bordering on extinct to active with an eruption thought to be “imminent”. The volcano is thought to have been last active some 174,000 years ago… Although an eruption may catch us unawares with often both tragic and catastrophic consequences, it is because we were unaware of the warning signs or chose to discount or ignore them.

Plate-tectonic driven Arc volcanism (WikiMedia)

What drives volcanism? Mostly plate tectonics and that is why it is so localised to near the edges of the tectonic plates. As one plate slides under another, it takes with it all the bedrock and sedimentary layers often containing vast quantities of water. As it is subducted, it goes deep into the Earth’s mantle where it is heated until it begins to melt. At those very high temperatures and pressures, water acts as a flux and speeds up the process of re-melting.

The melted remains of the subducted plate then ascends through the crust of the overlying plate until it breaches the surface in a volcanic eruption. Volcanism along the plate boundaries is often “grey” and explosive due to the high water content. While most eruptions, while lethal if too close, are relatively small, some may reach truly apocalyptic levels.

Hot Spot volcanism. As the Pacific Plate moves over the fixed hot spot, it breaks through to form a chain of islands from Midway to Hawaii itself. As the hotspot passes, the islands formed slowly sink back below the surface. (oregonstate.edu)

The other type of volcanism is driven by hot spots in the Earth’s mantle that may originate as far down below as the Earth’s core. Here, for reasons not fully understood, a region in the Earth’s mantle becomes much hotter and as it does, it rises to break through whatever crust lies above it. While the overlying plate may be re-melted in a manner similar to that induced by subduction and result in explosive, “grey” eruption, hot spot volcanism is usually “red” and effusive. Great examples of this are to be found in Iceland, the Azores or Galapagos Islands and the chain of islands from Midway to and including Hawaii.

So what types of eruptions are there and what causes them? This depends almost exclusively on the type of magma eventually erupted and how much volcanic gas it contains. Let us state this immediately – water is by far the most important of all volcanic gases as it is in solution with the melted rock, magma, in its superheated form. When it breaks surface, superheated water (or steam as it is often called) instantly flashes into steam and expands thousands of times in volume. This is the explosive power behind such eruptions as that of Mount St Helens in 1980 or Mount Pinatubo, the Philippines, in 1991.

As magma is formed in the depths, it is called juvenile magma. This magma is called basalticbecause of its composition and temperature. Not surprisingly, basaltic magma has the highest temperature of all erupted magmas. But if it does not immediately erupt, but remains in the crust at depth (usually 5 – 15 km) for a long period of time; hundreds, thousands or even tens of thousands of years, the magma cools and changes in chemical composition from basic to progressively more acidic types of evolved magmas – andesitic, daciticand finally rhyolitic. While basaltic magma is runny, as it evolves it becomes more “sticky”, viscous. Generally speaking, the more viscous the magma is and the higher the water content, the more explosive will any resulting eruption be.

The Birth of Surtsey and origin of the term Surtseyan eruption. (Uncredited image from the Web)

But things are never as simple as this. Not only are there further subdivisions in types of eruptions such as Peléean, Subplinian, Sub-glacial and sub-acquatic or Surtseyan. The magmas themselves are further subdivided into many different types depending on chemical composition. This has its origin in what type of rock was initially melted and what types of rock were encountered and re-melted on the way to the surface. The below table gives an idea of these subdivisions.

Classification and nomenclature of volcanic rocks according to their modal mineral contents using the QAPF diagram (based on Streckeisen, 1978, Fig. 1. uounbi.au).

Also, there are very few volcanoes that have only a single magma chamber. Most have several chambers, pockets, dikes and sills where magma at different stages of evolution has collected. Therefore, we usually refer to these collectively as the volcano’s magma reservoir. What makes prediction of volcanic eruptions so difficult is that we rarely know of these magma collection points and at what stage each is. Should a new magmatic intrusion occur into an already existing body of magma, that will be re-heated or re-mobilised and possibly as with the Laacher See eruption of c~12,900 BP result in a very large eruption.

Different types of (earthquake) tremor. Above each given type is its profile as it would appear on a Drum plot while below the audiogram spectrum of the same earthquake. (USGS)

So how do we get an idea of what is happening? We monitor the volcano! The simplest form is mechanical to acoustic – We “listen to” earthquakes and from their nature and progress we may deduce what is happening. We can also use satellite GPS and interferometry to get an idea if the ground is subsiding, stable or rising. The latter may indicate that there is an ongoing intrusion of magma, but it may be no more than snow and ice on the GPS antenna! Also, with the aid of high-precision lasers and reflectors, we can get a very accurate picture of how much and where a monitored volcano inflates. Then as amateurs, we avidly read every single paper on a given volcano we can get our hands on and this is where Volcanocafé excels! Together we are a formidable body of amateur volcano sleuths!

By now, the reader’s picture of what a volcano really is should have evolved from a simple drawing or cutaway to begin to resemble the above picture (courtesy VC reader DeepThought)

Here is an USGS video that explains what happens when a (long-)dormant stratovolcano reawakens: